The long-awaited National Museum of African American History and Culture took an important step forward yesterday with the selection of an architectural and design team.

The Smithsonian, which is overseeing the $500 million project on the Mall, chose Freelon Adjaye Bond/SmithGroup as the firm that will design the signature building across the street from the Washington Monument.

"Their vision and spirit of collaboration moved all members of the design competition jury," said the museum's founding director, Lonnie G. Bunch III. "I am confident that they will give us a building that will be an important addition to the National Mall and to the architecture of the city."

The museum's mission is to tell the African American story from the African origins, through slavery and emancipation, to politics, music, sports and spirituality. Its five-acre setting at Constitution Avenue between 14th and 15th streets NW makes it most likely the last major structure built along the historic Mall. The museum is expected to be completed in 2015.

"This is an incredible time for us as designers -- and this museum represents a unique opportunity to give form and substance to the powerful vision that has been established by the Smithsonian leadership," said Philip G. Freelon, president and founder of the Freelon Group.

David Adjaye, the lead designer, said the aim was to construct an edifice that spoke about celebration and praise. "We are celebrating an incredible journey and looking to the future," said Adjaye, 42, who is considered one of the leading architects of his generation. The Tanzanian-born, London-based architect also designed the Nobel Peace Center in Oslo.

The designer's fee will be determined through a contract process; each team that submitted a design proposal received $50,000.

The group's concept, which won the approval of a 10-person jury, shows a square building, held by four columns with an open first floor contained in a porchlike design. Two superstructures, which are shaped like crowns and inspired by an African headdress, top the entry-level porch. "Corona," like a crown jewel, is how Adjaye described the shape. On top of the crowns, which will hold most of the exhibition galleries, is a roof garden. The four-story design has several overlooks, taking advantage of the iconic sights along the Mall. It stands at 105 feet.

The base is built with stone, and the crown elements are bronze, porous enough to let in the natural light, and the material was chosen to pick up the different patinas of day and night. The first-floor space is 100 feet wide with no columns, and the vastness is broken up by 12- to 15-foot hanging wooden slats. The space is also marked by an opening in the floor, which will allow the music from the bottom floor to drift throughout the public space. The story of African American music is planned for that lower floor.

The entrances are on Constitution Avenue and Madison Drive, on the Mall side, bringing all visitors into the open first floor. On the Constitution side is a canal, representing the Washington Channel, where slaves and supplies were transported.

Freelon, the architect of the Reginald F. Lewis Museum of African American History and Culture in Baltimore, said the team was honored to have been chosen. At the briefing, he defended the boxy look of the exterior. "Our scheme is quite soaring from the interiors. The space suggests uplift," he said. Freelon, 56, is based in Durham, N.C., and is a recipient of the 2009 Thomas Jefferson Award for Public Architecture from the American Institute of Architects.

Currently, the members of the winning team are all designing branch libraries in Washington. Freelon Bond developed the pre-design study for the Smithsonian, which outlined what spaces were needed in the museum, and presented the plan to all the competing teams. J. Max Bond Jr., the dean of African American architects and one of the principals on the planning phase, died in February of cancer. His company, Davis Brody Bond, the recipient of more than 100 major design awards, is the designer for the memorial museum at the World Trade Center site, and did the Birmingham Civil Rights Institute.

The D.C.-based SmithGroup has worked on the National Museum of the American Indian and several other Smithsonian projects, as well as the International Spy Museum and the Sculpture Garden Pavilion at the National Gallery of Art.

Smithsonian Secretary G. Wayne Clough emphasized the importance of how the design related to the monument and the museum's materials. "It will tell an essential part of the American story," he said at a news conference at the Castle, the Smithsonian's first structure, completed in 1855.

The museum, which was approved by President George W. Bush in 2003, is a joint public-private project with half the funds coming from Congress.

The estimated start of construction is 2012. In the meantime the museum staff has been collecting artifacts, including the 5,000 photographs of H.C. Anderson in Mississippi and thousands of items from the Black Fashion Museum, and it is seeking a slave cabin. The museum is currently sponsoring an exhibition of the Scurlock Studio, a family photography business in Washington for most of the 20th century, at the National Museum of American History, and has launched an online museum.

The Freelon group's concept, said Bunch, will "help make manifest the dreams of many generations on the Mall."